1. Field
Exemplary embodiments relate to measurement technology, and more particularly, to optical measuring systems for measuring the geometrical parameters of nano-objects and methods of measuring a critical size (CS).
2. Description of the Related Art
In the current production of semiconductor chips, achievements in microlithography have resulted in the trend to a reduction of the critical size (CS) of articles. Herein, the term “critical size” means a characteristic size of a nano-structure, which size is of a certain interest and is equal to some tens of nanometers. At present, the minimum CS of the nano-structure is about 30 nanometers and it is anticipated that in the future, the minimum CS may be reduced to about 20 nanometers.
The mass production of semiconductor structures having such small CS requires more strict standards of precision and reliabilities of the measuring equipment, and also requires improvement in the speed and cost of a measuring process. Thus, the existing measuring methods based on use of the scanning electron microscope (SEM) and an atomic-powered microscope (APM) are too slow and expensive, especially at a stage of rejecting chips with known topologies of which characteristic sizes of the structure differ just slightly from prescribed sizes. For such measuring, various optical methods have been developed based on an ellipsometry technique (see “Handbook of Ellipsometry”, Harland G. Tompkins, Eugene A. Irene) and scatterometry (see Petre Catalin Logofatu et. al., Rom. Journ. Phys., Vol. 55, Nos. 3-4, P. 376-385, Bucharest, 2010), in particular, a widely known method “optical critical dimension” (OCD) (see Ray J. Hoobler and Ebru Apak, Proceedings of SPIE, Vol. 5256, 23rd Annual BACUS Symposium on Photomask Technology) which allows to distinguish a CS of a semiconductor structure smaller than a Rayleigh resolution limit.
Each existing optical analysis method has both advantages and drawbacks.
The OCD method is based on the dependence of the reflectivity factor of a subundular structure on the CS, on the wavelength and the sight angle of the incident radiation. Generally, two variants of application of the OCD method are used. A first variant is based on, at the fixed wavelength of the incident radiation, the dependence of the reflectivity factor on the angle of sight (scanning by angle). A second variant is based on, at the fixed angle of sight of the radiation, the dependences of the reflectivity factor on the wavelength (scanning by wavelength). In practice, the second variant measures the spectrum of the incident and reflected radiation, and, based on the results of these measurements, the dependence is determined between the reflectivity factor and the wavelength. The measured dependence is compared with the calculated dependences determined at various values of the CS. The best coincidence of the measured and calculated curve gives a required value of the CS.
The OCD method is commonly used in a semiconductor production. However it is not applicable to carry out the analysis of non-periodic (non-cycle) structures, structures with small set of periods (cycles) or structures consisting of one or several isolated objects.
The method of “optical microscopy scanning through focus” (TSOM, Through-focus Scanning Optical Microscopy) (see Attota, R., Silver, R. M., and Barnes, B. M., “Optical through-focus technique that differentiates small changes in line width, line height, and sidewall angle for CD, overlay, and defect metrology applications,” Proc. SPIE 6922, 6922OE-1-13, 2008), based on the analysis of low-contrast (defocused) images of an examined object generated by means of a microscope at scanning an object along an optical axis, enables analyzing the non-cycle and isolated objects. In the framework of TSOM, a mechanical scanning system, which ensures shifting the examined object along the focus with precision of some tens of nanometers, is one of core elements and, at the same time, it is the most vulnerable element, in the sense of reliability, of the TSOM measuring installation. Requirements for minimization of a necessary scanning step and for object positioning accuracy along the focus increase at a reduction of the characteristic sizes of the object, that, in case of vibrations, may reduce precision of measuring and reliability of all measuring systems as a whole. Further, the mechanical scanning method restricts the measuring quickness which is important in a set of practically important measuring problems of the semiconductor production. In this connection, the inspection methods which do not require mechanical scanning of a sample or of an individual element of the measuring system possess essential advantage.